Low base number calcium sulfonates are generally produced by the reaction of sulfonic acid with calcium hydroxide or calcium oxide, utilizing a promoter such as an alkanol. They can also be produced from sodium sulfonate by the use of calcium hydroxide or oxide and calcium chloride. Such sulfonates may be used as highly valued additives for lubricating oils such as passenger car, diesel, and marine engine lubricants. They may be further processed into overbased sulfonates, which have higher base numbers and are also used as additives for specialty lubricating oils.
When calcium sulfonate is derived from sulfonic acid, no chloride is needed, but the concentration of the final product is limited by the concentration of the sulfonic acid. In the case of natural petroleum sulfonic acid, concentration is typically less than commercially desired. Concentrating sulfonic acid itself is difficult due to its high corrosivity.
When calcium sulfonate is made from sodium sulfonate, chloride is required to make the reaction proceed. This leads to residual contaminating chloride in the final product. The sodium sulfonate is concentrated to the required concentration using a solvent extraction process prior to conversion to the calcium product, since calcium sulfonate is more difficult to concentrate by this method.
A number of methods have been disclosed for the production of low base number calcium sulfonate.
U.S. Pat. No. 5,804,094 teaches a method of producing a low base number calcium sulfonate of greater than 500 molecular weight using carboxylic acid and a high base number calcium sulfonate.
U.S. Pat. No. 5,789,615 teaches the use of staged addition of calcium hydroxide to sulfonic acid to produce a low viscosity, low haze product without the use of promoters, especially without the use of chloride. The calcium hydroxide is added in two or more steps, with 30–180 minutes heat soak after each step.
U.S. Pat. No. 4,615,841 describes a method of producing calcium sulfonates in the presence of an alkanol.
U.S. Pat. No. 4,279,837 teaches the preparation of alkaline earth metal salts of alkyl benzene sulfonic acids by neutralization of the acid using an oxyalkylate as a promoter, thus also producing a chloride free calcium sulfonate.
U.S. Pat. No. 3,719,596 describes a method of producing calcium sulfonate in which the reaction mixture is made acidic and then basic again using an alkanolamine.
U.S. Pat. No. 2,779,784 teaches a method of producing calcium sulfonate in which sulfonic acid is neutralized with calcium hydroxide at 220° F. to 390° F. (104° C. to 199° C.), in the presence of ½ to 10 parts water per part calcium hydroxide. This would correspond to between 0.12 and 2.4 mol water per mol calcium hydroxide.
It would be advantageous to produce low base number calcium sulfonates, that are free of residual chlorine and easily concentrated, via a process suitable for use in a continuous reactor that can also produce products with a low viscosity.